A technique called multimedia broadcast and multicast service (MBMS) in radio communication between a mobile station and a base station is currently available. MBMS is a technique for efficiently distributing multimedia contents such as moving images and music over a cell phone network. Multicast communication is given as one of features of MBMS. In the multicast communication, a base station transmits the same data to a plurality of mobile stations using the same frequency band. The multicast communication has been studied for mainly a service of distributing moving images and music. Since the multicast communication allows distribution of information in a lump to a plurality of mobile stations by using the same frequency band, such a merit is obtained that resources are more effectively utilized than in one-to-one communication (unicast communication).
Japanese Laid-open Patent Publication No. 2005-502135, Japanese Laid-open Patent Publication No. 10-257100, and Japanese Laid-open Patent Publication No. 2008-252347 disclose related techniques.